I am using ChartEventMarker. I attach events to it like "PositionChanged,MouseClick ". Both work. However MouseUp event is not fired at all. When should I expect the event to be fired? Why is it not triggering?

We tested the events with ChartEventMarkers, but weren't able to reproduce this issue. At least PositionChanged, MouseClick, MouseDown and MouseUp -events were working correctly. MouseUp -event triggers the moment a mouse button is released. It then checks if the mouse is above the component, in this case the marker. So one reason why it might not trigger is that the mouse is not above the marker. This is easy to check if MoveByMouse -property is set false: press mouse button above the marker, move mouse away, release button. Do the same without moving the mouse away.

Here is an example code which adds a marker with working MouseUp -event:

I checked this again and found out that there is nothing wrong in your code, it is the LightningChart version that causes it. In some 8.2 versions MouseUp -event was bugged with some components such as annotations and event markers. This was fixed in later versions of LightningChart, which explains why it worked in my tests.

Therefore, I see two possible options to make the event work in your application:
1. Updating LightningChart to the newest version.
2. If the above is not an option, a workaround has to be used. One way is to use MouseUp -event for the chart component, which seems to work properly. It triggers whenever a mouse button is released as long as the chart has focus. You can use for example a boolean variable to make it to work with marker's MouseDown -event. Here is an example:

In this example whatever happens in MouseUp -event, is executed only if the marker is clicked first. One thing to note is that in this case the mouse cursor can be anywhere when the button is released (doesn't have to be above the marker).